words per minute
20
DbruChttrj
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Animals are great builders, using simple materials but employing sound engineering principles. Man has long used suspension bridges made of ropes of steel and concrete, which bear loads in tension. The webs of spiders provide the most vivid examples of animal structures built entirely in tension.
Spider-web silk is so strong that many species of birds use it to build their nests. It has a stiffness index which is 30 times less than of steel, yet has a tensile strength one ane and a half times as much as that of steel. Unlike steel, silk stretches to more than twice its original length before breaking. These qualities, it is now known, are owing to silk being a protein polymer composed of crystalline regions interspersed with regions of randomly arranged chains.
Beavers are natural engineers. They use tree branches to construct dams. the divided end being pressed into the walls: this apparently serves to collect stress from a broad area and to focus it into a single trunk embedded in the substrate of the stream. The dams of the beaver have an angle of 45 degree on the downstream side. This enables the structure to bear4 not only its own weight acting downward but also resist the lateral pressure of water acting on the upstream side.
Constructing a very thick wall is uneconomical. Strength could also be provided to a thin wall by buttresses or by making the base wide. The nest of the termite Amitemes meridionalis is more than two meters high with a side base which tapers at the top. This ensures greater resistance to shear forces.